The present invention relates to a film-forming apparatus and a film-forming method adapted for forming a thin film on a high-molecular film as a substrate by a gas-phase reaction using plasma.
High-density magnetic recording media such as a metal vapor deposition tape used as a high-quality video tape, a hard disk or high-density floppy disk used as a large storage capacity storage for a computer, etc., and other magnetic recording media have been further developed into higher-density media recent years.
For example, there are used a metal vapor deposition tape for a digital video camera and a floppy disk having a recording capacity of 100 MB! or a hard disk having a recording capacity of the order of GB (gigabyte) as a storage device for a computer.
Because the floppy disk uses a web-like high-molecular film as its high-molecular substrate, the productivity of the floppy disk is very high compared with those of a hard disk and an optical disk which use a fixed substrate in the producing process, As a method of improving the recording density of the floppy disk more greatly, there is thought of a configuration in which a metal such as CoNi, CoNiPt, CoCrPt, CoCrTa, CoCrPtSi, SmCo, etc., is formed as a magnetic film.
Further, a material of a protective film for protecting the metal magnetic film is required to have a characteristic excellent both in hardness and in slidability. Examples of the material having such a characteristic include SiNr, SiO.sub.2, TiN, BN, CN, a-C, diamond-like carbon (DLC), etc.
In this case, as a method of forming a protective film by using the aforementioned material, there are known a method of forming a carbon protective film by sputtering and a method of forming a protective film by plasma CVD. Because, in comparison on film-forming speed, the sputtering method is extremely slower than the plasma CVD method, it is preferable to employ the plasma CVD type method, taking the productivity or the like into consideration.
In the plasma CVD method, it is known that the amount of the decomposed reaction gas increases to thereby increase the film-forming speed as the number of electrons in plasma increases and as the density of the plasma increases, because molecules of the reaction gas are excited and ionized to cause a decomposing reaction by the collision of the reaction gas with electrons in the plasma so that a thin film is formed on a substrate by a gas-phase reaction, that is, a protective film is formed.
As a plasma generating method using plasma CVD, there are known a DC arc discharge method, a high-frequency discharge method, etc. The density of plasma varies greatly in accordance with the plasma generating method, The plasma density in the high-frequency discharge method is in a range of about from the order of 10.sup.9 /cc! to the order of 10.sup.10 /cc! whereas the plasma density in the DC arc discharge method is in a range of about from the order of 10.sup.11 /cc! to the order of 10.sup.12 /cc!.
Accordingly, in comparison on plasma density, the DC arc discharge method is superior in that film-forming can be performed at a high speed. However, plasma generated by the DC arc discharge method is called "equilibrium plasma" and is very high in ion temperature so that it is difficult to produce good-quality magnetic recording media because a high-molecular film is thermally damaged by the radiant heat of plasma in the case where tho high-molecular film is used as a substrate.
For example, in "DC Arc Discharge CVD Apparatus" disclosed in U.S. Pat. No. 5,232,791 or in "DC Arc Discharge CVD Apparatus" disclosed in Japanese Patent Postexamination Publication No. Hei-7-51753, the density of plasma is high and these apparatuses are therefore suitable for high-speed film-forming with respect to this point. It is however known that, when a high-molecular film such as a PET film, a PEN film, an aramid film, a polyimide film, etc., is used as a substrate, the substrate is seriously damaged by the radiant heat of plasma because the temperature of the plasma is high. Accordingly, these apparatuses are not suitable for production of good-quality magnetic recording media.
On the other hand, when discussion was made upon "High-frequency CVD Apparatus" disclosed in U.S. Pat. No. 5,360,483 and "High-frequency CVD Apparatus" disclosed in Japanese Patent Postexamination Publication No. Hei-7-100857 using the high-frequency discharge method, there was found a problem that the density of plasma is low and the film-forming speed is low as described above.
In the "High-frequency CVD Apparatus" described in the above U.S. Pat. No. 5,360,483, because a reaction tube is used, the film thickness distribution in the widthwise direction is poor so that the apparatus is not suitable for a continuous substrate such as a high-molecular film. On the other hand, in the "High-frequency CVD Apparatus" disclosed in Japanese Patent Postexamination Publication No. Hei-7-100657, because slag is accumulated between a film-forming drum and an electrode in the case where film-forming is performed continuously, the surface of the substrate is injured easily so that it is not preferable to use the apparatus in a process of forming a film on a long-size high-molecular film.
Judging from the above description as a whole, what is meant is that nonequilibrium and low-temperature plasma is required in the case where a high-molecular film is used as a substrate and that a plasma source satisfying the condition of high plasma density is required for improving productivity.
Therefore, employment of a microwave ECR plasma CVD apparatus using micro ECR (electron cyclotron resonance) as a plasma generating method is thought of. In this case, it is known that generated plasma is low-temperature nonequilibrium plasma and that the density of the plasma is in a range of about from the order of 10.sup.11 /cc! to the order of 10.sup.12 /cc!. In this regard, film-forming can be performed both at a low temperature and at a high speed so that the merit of the DC arc discharge method and the merit of the high-frequency discharge method can be made consistent with each other.
FIG. 3 shows a conventional microwave ECR plasma CVD apparatus 100 (hereinafter simply referred to as "apparatus 100") as described above. As understood from FIG. 3, the apparatus 100 has a vacuum chamber 110 in which the inside gas is drawn out from an outlet 120. In the inside of the vacuum chamber 110, there are provided a delivery portion 140 for delivering a substrate 130 to a protective-film-forming region in order to carry the substrate 130, an electrode roller 130 for applying a bias voltage to the substrate 130, a cooling drum 160 for cooling the substrate 130 in the protective-film-forming region, a pass roller 170, a wind-up portion 180 for winding up the substrate 130 after the formation of the protective film, and a reaction gas introducing portion 190 for introducing a protective-film-forming gas into the vacuum chamber 110.
In the outside of the vacuum chamber 110, there is provided a microwave generating portion 200 for generating microwave. The generated microwave is propagated into a plasma generating chamber 230 through a microwave introducing portion 210 and a microwave introducing window 220. A plasma-generating inert gas is introduced into the plasma generating chamber 230 from an inert gas introducing portion 240.
In the outside of the plasma generating chamber 230, there is provided a plurality of current coils 250 to form a magnetic field for generating plasma. By the configuration, electron cyclotron resonance is applied to the introduced inert gas so that the substantially same number of nonequilibrium low-temperature plasma au that obtained by the arc discharge method is generated.
Because the apparatus 100, however, uses the large-size current coils 250 in order to form a magnetic field for ECR, the distribution of the magnetic field thus formed becomes uneven. As a result, there arises a serious disadvantage that film-forming cannot be performed over a large area and, particularly, the film thickness distribution in the widthwise direction becomes poor.
Although, in this case, such a configuration in which a number of current coils 250 are used so that the magnetic field formed in the vacuum chamber 110 is made uniform may be employed, there arises a problem that the cost of production of the apparatus 100 as a whole is increased. Furthermore, even if the magnetic field is intended to be made even as described above, the degree of evenness is insufficient to make the film thickness distribution good with respect to the high-molecular substrate in the form of a web-like high-molecular film.
Furthermore, in the case where microwave is introduced into the vacuum chamber 110, a protective film and slag which is generated in film-forming are not transparent and are deposited on the microwave introducing window 220 to contaminate the microwave introducing window 220 because there is employed a configuration in which the dielectric microwave introducing window 220 is interposed in a microwave propagation path. As a result, the output of microwave supplied for plasma generation is lowered. Accordingly, when a long-size film is used as the substrate 130, the film thickness in the lengthwise direction of the substrate 130 varies largely so that the film thickness cannot be made even. As a result, sufficiently good quality cannot be obtained.